A new type of mutation--bromodeoxyuridine (BUDR) dependence--has been discovered in mammalian cells in vitro. The mutant cells, derived from a Syrian hamster melanoma line, grow poorly unless BUDR is added to the medium at concentrations which kill normal cells. The BUDR dependent cells are able to grow with at least 99.8% of the thymidine in DNA replaced by BUDR. The existence and properties of these cells suggest that they may have undergone alterations in the systems involved in DNA synthesis, DNA repair, RNA synthesis, and cellular interactions. We plan to study those systems in the BUDR dependent cells in which there may have been some alteration. Specifically, we will examine DNA polymerase and RNA polymerase activities, DNA repair, and cell membrane properties. The demonstration of alterations within these systems should help to explain BUDR dependence and, more significatly, should provide evidence on the enzymes and factors which are involved in the replication, repair, and transcription of DNA and in the determination of cell surface properties in mammalian cells. The experiments will involve the techniques of column and paper chromatography, enzyme assay, density gradient centrifugation, and cell culture.